Reprogrammable electrical fuse

ABSTRACT

The present invention provides a reprogrammable electrically blowable fuse and associated design structure. The electrically blowable fuse is programmed using an electro-migration effect and is reprogrammed using a reverse electro-migration effect. The state (i.e., “opened” or “closed”) of the electrically blowable fuse is determined by a sensing system which compares a resistance of the electrically blowable fuse to a reference resistance.

REFERENCE TO PRIOR APPLICATIONS

This application is related to co-pending U.S. patent application Ser.No. 10/908,245, filed on May 4, 2005, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to fuses included withinsemiconductor structures. More particularly, the present inventionprovides an electrical fuse that can be reprogrammed using a reverseelectro-migration effect, and associated design structure.

2. Related Art

As is known in the art, many modern semiconductor integrated circuitsinclude fuses to protect sensitive parts during the manufacturingprocess, and for the activation of redundant circuits, such as redundantmemory cells in the case of Dynamic Random Access Memories (DRAMs).There are typically two types of fuses, a laser-blowable fuse, and anelectrically (e.g., current) blowable-fuse. Electrically blowable fusesprovide an advantage over laser-blowable fuses in terms of size.

With laser blowable fuses, the fuses are typically formed at or near thesurface of the integrated circuit. A laser beam striking the fusematerial renders the fuse non-conductive, thereby inhibiting currentfrom flowing through the fuse. Although laser blowable fuses arerelatively simple to fabricate, there are disadvantages associated withthem. For example, laser blowable fuses tend to be surface oriented,which places a limitation on the design of the integrated circuit.Further, laser blowable fuses tend to occupy a large amount of space onthe surface of an integrated circuit, since adjacent fuses or devicesmust not be placed too close to the fuse or risk being inadvertentlydamaged by the laser beam during the fuse blowing operation.

Electrically blowable fuses, on the other hand, do not have to be placedat or near the surface of the an integrated circuit. Accordingly, theygive designers greater latitude in fuse placement. In general,electrically blowable fuses tend to be smaller than laser blowablefuses, which render them highly suitable for use in modern high densityintegrated circuits. Further, electrically blowable fuses have a highprogramming speed compared to conventional laser blowable fuses.

Various means have been used in the past to blow electrically blowablefuses. One recently used technique for opening the connection at thefuse employs the electro-migration effect, which has long beenidentified as a major metal failure mechanism. Electro-migration is theprocess whereby the ions of a metal conductor move in response to thepassage of a high density current flow though the conductor. Such motioncan lead to the formation of “voids” in the conductor, which can grow toa size where the conductor is unable to pass current. One can takeadvantage of the electro-migration effect to selectively open up metalconnections (e.g., fuses) at desired locations within an integratedcircuit.

One limitation of such electrically blowable fuses is they can beprogrammed only once (e.g., from a state “1” (conducting) to a state “0”(non-conducting)). In other words, once an electrically blowable fusehas been opened using the electro-migration effect it can not be closedagain. Therefore, to reprogram or reconfigure an integrated circuit,redundant electrically blowable fuses and complicated supportingcircuitry would be necessary.

Studies have been made regarding the healing of electro-migrationrelated damage using a current reversal method. Evidence of such healinghas been reported by E. Castano, et al, in a paper entitled “In SituObservation of DC and AC Electro-migration in Passivated Al Lines,”Applied Physics Letters, Volume 59, Issue 1, Jul. 1, 1991, pp. 129-131.In this paper, it was shown that void size could be decreased byapplying current stress in a reverse direction. As depicted in FIG. 1,for example, it was found that the average void size was reduced from5.0 μm² (point A) to 1.5 μm² (point B) in less than one hour. A similarstudy was presented by J. Tao, et al. in a paper entitled, “AnElectro-migration Failure Model for Interconnects under Pulsed andBi-directional Current Stressing,” IEEE Trans on Electron Devices, Vol.41, No. 4, April 1994, pp. 539-545. In this paper, it was shown that theresistance of a conductor made of Al/Si could be altered back and forthduring forward and reverse current stressing as shown in FIG. 2. Theseand other such studies, however, have not provided a solution to the“programming only” nature of electrically blowable fuses that areprogrammed using the phenomenon of electro-migration.

SUMMARY OF THE INVENTION

The present invention provides an electrical fuse that can bereprogrammed using a reverse electro-migration effect, and associateddesign structure. Electro-migration is used to open a connection in theelectrical fuse, while reverse electro-migration is used to subsequentlyclose the opened connection. A programming/reprogramming circuit isprovided to enable the use of such a reprogrammable electrical fuse.

An aspect of the present invention is directed to a design structureembodied in a machine readable medium used in a design flow process, thedesign structure comprising a fuse system, the fuse system comprising:an electrically blowable fuse; circuitry for programming theelectrically blowable fuse using an electro-migration effect; circuitryfor reprogramming the electrically blowable fuse using a reverseelectro-migration effect; a reference resistance provided by a portionof the electrically blowable fuse; and circuitry for determining a stateof the electrically blowable fuse by comparing a resistance of theelectrically blowable fuse to the reference resistance.

Another aspect of the present invention is directed to a designstructure embodied in a machine readable medium used in a design flowprocess, the design structure comprising a fuse system, the a fusesystem comprising: means for programming an electrically blowable fuseusing an electro-migration effect; means for reprogramming theelectrically blowable fuse using a reverse electro-migration effect;means for providing a reference resistance using a portion of theelectrically blowable fuse; and means for determining a state of theelectrically blowable fuse by comparing a resistance of the electricallyblowable fuse to the reference resistance.

Another aspect of the present invention is directed to a designstructure embodied in a machine readable medium used in a design flowprocess, the design structure comprising an integrated circuit, theintegrated circuit comprising a fuse system, the fuse system comprising:an electrically blowable fuse; circuitry for programming theelectrically blowable fuse using an electro-migration effect; circuitryfor reprogramming the electrically blowable fuse using a reverseelectro-migration effect; a reference resistance provided by a portionof the electrically blowable fuse; and circuitry for determining a stateof the electrically blowable fuse by comparing a resistance of theelectrically blowable fuse to the reference resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of this invention will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings in which:

FIGS. 1-2 depict the healing of electro-migration related damage using acurrent reversal method.

FIGS. 3A-3C, 4A-4C, 5A-5C, and 6A-6C illustrate the operation of areprogrammable electrical fuse in accordance with an embodiment of thepresent invention.

FIGS. 7A-7C illustrate the operation of a reprogrammable electrical fusein accordance with another embodiment of the present invention.

FIG. 8A depicts programming and reprogramming circuitry for areprogrammable electrical fuse in accordance with the present invention.

FIG. 8B depicts another embodiment of programming and reprogrammingcircuitry for a reprogrammable electrical fuse in accordance with thepresent invention.

FIG. 9 depicts a sensing circuit for a reprogrammable electrical fuse inaccordance with an embodiment of the present invention.

FIG. 10 depicts a sensing circuit for a reprogrammable electrical fusein accordance with another embodiment of the present invention.

FIG. 11 depicts an illustrative physical layout of a reprogrammableelectrical fuse system in accordance with an embodiment of the presentinvention.

FIGS. 12-16 depict an analytical model for illustrating the predictedelectro-migration behavior in a tapered structure in accordance with theconcepts of the present invention.

FIG. 17 depicts a block diagram of a general-purpose computer systemwhich can be used to implement the design structure described herein.

FIG. 18 depicts a block diagram of an example design flow.

The drawings are merely schematic representations, not intended toportray specific parameters of the invention. The drawings are intendedto depict only typical embodiments of the invention, and thereforeshould not be considered as limiting the scope of the invention. In thedrawings, like numbering represents like elements.

DETAILED DESCRIPTION

The general operation of a reprogrammable electrical fuse 10, hereafterreferred to as an “e-fuse 10” in accordance with an embodiment of thepresent invention is depicted in FIGS. 3A-3C. In this example, thee-fuse 10 has a “dog-bone” shape to facilitate programming andreprogramming operations. As depicted in FIG. 3A, the e-fuse 10 includesa first conductive body region 12, a second conductive body region 14,and a conductive neck region 16 extending between the first and secondbody regions 12, 14. During programming, as shown in FIG. 3B, a void 18forms in the neck region 16 of the e-fuse 10 in response to theapplication of a current I. This occurs because of the so-called“crowding effect,” which is caused by the flow of electrons from alarger cross-sectional area (e.g., body region 12) into a smallercross-sectional area (e.g., neck region 16). This leads to a largetemperature gradient, which expedites the electromagnetic effect. Theformation of the void 18 causes the resistance of the e-fuse 10 toincrease significantly. It should be noted that the neck region 16 mustremain conductive to allow a reverse current flow required forreprogramming of the e-fuse 10.

During reprogramming, as shown in FIG. 3C, the void 18 is refilled andpushed toward the larger cross-sectional area of the body region 12 inresponse to the application of an oppositely directed current I, therebyrestoring the resistance of the e-fuse 10. This occurs because of aso-called current “de-crowding effect.” If too much current stress isapplied in the reverse direction, however, the void 18 may start toincrease in size again at a different location. To this extent, to makethe e-fuse 10 of the present invention reprogrammable, one must:

-   (1) Provide a structure that allows metal to migrate in a    controllable manner in both forward and reverse current flow    directions;-   (2) Create a large resistive ratio change; and-   (3) Provide a reference resistance R_(ref) to determine the state of    the e-fuse 10. Prior to programming (and after reprogramming) the    reference resistance R_(ref) is higher than the resistance of the    e-fuse 10. After programming, the reference resistance R_(ref) is    much lower than the resistance of the e-fuse 10.

Different voltages and currents may be applied to the e-fuse 10 toperform programming and reprogramming. During programming, theresistance of the e-fuse 10 will rise higher than the referenceresistance R_(ref), while during reprogramming the resistance of thee-fuse 10 will drop lower than the reference resistance R_(ref).

The present invention provides a sensor circuit (described in greaterdetail below) to sense the change of resistance of the e-fuse 10 andlatch the results into a corresponding register. One referenceresistance can be shared by a bank of e-fuses 10 to save power and areaoverhead. In this case, sensing can be done in sequential manner, forexample, during a power-on sequence to read the bank of e-fuses 10, andthe results stored in one or more registers. The stored results can beused to provide information regarding programming state. The registerscan comprise a small cache memory like that of DRAMs, or localregisters.

As shown in FIG. 4A, an illustrative e-fuse 10 has a “dog-bone” shapewith a taper angle θ of about 45 to 75 degrees and a neck region 16 witha width/body ratio from about 1/10 to 1/3, depending on the width of thebody regions 12, 14. As will be presented in greater detail below, thetaper facilitates the programming/reprogramming of the e-fuse 10.

Cross-sectional views of the e-fuse 10 taken along line 4B-4B and 4C-4Care illustrated in FIGS. 4B and 4C, respectively. A uniform thickness ofa barrier film formed of a material such as titanium (Ti), tantalum(Ta), tungsten (W), titanium nitride (TiN), or a combination thereof isdeposited in a conventional manner at the bottom of the e-fuse 10 toform a barrier layer 20. Due to the reduced surface area within the neckregion 16, the deposited barrier film will be much thicker in the neckregion 16 as compared to the body regions 12, 14. The thicker barrierlayer 20 in the neck region 18 shrinks the cross-section of the neckregion 16 in the third (i.e., Z) dimension. This increases the currentdensity inside the neck region 16, thereby enhancing theelectro-migration effect in the e-fuse 10.

A metal material 22, such as aluminum (Al), copper (Cu), aluminum-copper(Al/Cu) alloy, or other suitable metal material susceptible toelectro-migration, is then deposited and planarized (e.g., usingchemical-mechanical-polishing (CMP)). A depletion region d1 is formed atthe surface of the body regions 12, 14. A dielectric material (notshown) is deposited to cap the top surface of the e-fuse 10.

The sidewall 24 of the e-fuse 10 comprises a barrier liner formed of amaterial such as Ti, Ta, W, TiN, or a combination thereof. Otherconductive materials such as doped poly-silicon or a silicided diffusionregion can also be used. The barrier liner can be used to provide thereference resistance R_(ref) described above with regard to theprogramming/reprogramming of the e-fuse 10. The material of the barrierliner is not sensitive to the electro-migration effect and has aresistance value higher than that of the e-fuse 10 prior to programming(and after reprogramming) and a resistance value much lower than that ofthe e-fuse 10 after programming. The material of the barrier liner ispreferably compatible with back end of line (BEOL) metallizationprocesses to limit processing costs.

After programming, as shown in FIGS. 5A-5C, a void 18 is created in theneck region 16 of the e-fuse 10 due to the electro-migration effect. Asa result, the resistance of the e-fuse 10 is drastically increased, eventhough the void 18 may only be located partially within the neck region16. Programming conditions (e.g., voltage, current, temperature, etc.)are controlled so that that a desired void 18 size is formed. If thevoid 18 size is too small, the e-fuse 10 will be under-programmed. Ifthe void 18 size is too large, it may not be possible to reprogram thee-fuse 10. Neither of these conditions is desirable.

As shown in FIG. 5B, during the programming of the e-fuse 10, metalmigrates from the neck region 16 of the e-fuse 10 toward the body region14 and accumulates to a depth d2. As a result, this area of the e-fuse10 has a higher atomic density and is more stressed than beforeprogramming. The migration of the metal results in the creation of avoid 18 located at least partially within the neck region 16 of thee-fuse 10. Different degrees of programming can be used to createdifferent sized voids 18′, 18″, etc., with different depths, resultingin different resistance values for the programmed e-fuse 10. Theresistance of the e-fuse 10 after programming is much greater than thereference resistance R_(ref).

During the programming of the e-fuse 10, high-voltage and high-currentare applied to the e-fuse 10 at room or high temperature (e.g., 100 to250° C.) to “open” the e-fuse 10 in a relatively short period of time.Care must be taken, however, to ensure that at least some of the metalmaterial 22 remains within the neck region 16 to allow a reverse currentto be applied during a subsequent reprogramming of the e-fuse 10.

The reprogramming of the e-fuse 10 is illustrated in FIGS. 6A-6C. Aswith programming, the reprogramming of the e-fuse 10 is carried outunder high-current, voltage and at room or high temperature, but in theopposite direction. As shown, excessive metal that accumulated on thebody region 14 of the e-fuse 10 migrates toward the neck region 16 andat least partially fills the void 18. It may be desirable to performin-situ monitoring during reprogramming to minimize the depth of thevoid 18 inside the neck region 16. The resistance of the e-fuse 10 afterreprogramming is once again much lower than the reference resistanceR_(ref).

Another embodiment of the present invention is depicted in FIGS. 7A-7C.As shown, a reprogrammable e-fuse 30 can be formed using a conductivedouble-layer metal structure 32. In particular, a top metal layer 34 ofthe double-layer metal structure 32 can be formed using a metal materialthat is susceptible to electro-migration, while the bottom metal layer36 of the double-layer metal structure 32 can be formed using a metalmaterial that is much less susceptible (or not susceptible) toelectro-migration. For example, since pure copper (Cu) is at least 2 to4 times more susceptible to electro-migration than pure aluminum (Al) orcertain alloys of Al, the top metal layer 34 can be formed of copper,while the bottom metal layer 36 can be formed of Al or an alloy thereof.In another embodiment of the present invention, the metal layers 34 and36 can be reversed such that the metal material that is susceptible toelectro-migration is located below the metal material that is much lesssusceptible (or not susceptible) to electro-migration. The metalmaterial that is susceptible to electro-migration can also be sandwichedbetween layers of, or surrounded by, the metal material that is muchless susceptible (or not susceptible) to electro-migration.

During programming of the e-fuse 30, as shown in FIG. 7B, a void 38 isformed in the top metal layer 34, which increases the resistance of thereprogrammable e-fuse 30 such that it is much greater than a referenceresistance R_(ref) of the e-fuse 30. During reprogramming, as shown inFIG. 7C, the void 38 is at least partially refilled and the resistanceof the e-fuse 30 is reduced. Any suitable bi-layer or multi-layer metalstructure can be used to form the reprogrammable e-fuse 30. The bottommetal layer 36 can also be used to provide the reference resistanceR_(ref) instead of using a barrier liner as detailed above.

A first embodiment of programming and reprogramming circuitry 40 for areprogrammable e-fuse 10 in accordance with the present invention isdepicted in FIG. 8A. During programming, the control pin “F” is set tohigh, so that the two nMOS devices N11 and N12 are on, but the other twonMOS devices N10 and N13 are off. A programming current with a presetcurrent pulse height and width is applied to the e-fuse 10 from net A tonet B. Similarly, during reprogramming, the control pin “F” is set tolow, so that the two nMOS devices N11 and N12 are off, but the other twonMOS devices N10 and N13 are on. A reprogramming current with anotherpreset pulse height and width is applied to the e-fuse 10 element in theopposite direction, from net B to net A.

Another embodiment of programming and reprogramming circuitry 40′ for areprogrammable e-fuse 10 in accordance with the present invention isdepicted in FIG. 8B. Here, instead of using nMOS devices for switches,any conventionally available switch such as a transmission gate device,MEMS switches, etc., can be used. In this example, four transmissiongate devices T11, T12, T13, and T14 are used to program the e-fuse 10.The advantage of using transmission gate devices over nMOS device iswell known in the art, and thus will not be described further. In thisexample, a “disable” control signal is used when the fuse programmingprocess is finished. This will switch off the power generator 16 as wellas two pull down transmission gate devices T12 and T13 so that internalnodes A and B become floating. Two OR gate devices OR1 and OR2 are usedto provide forward, reverse, and enable (or disable) programmingcontrol. The circuit operation is similar to that described of FIG. 8A,except that only one power generator 16 is provided, which can be usedfor either forward ore reverse programming.

An illustrative sensing circuit 50 for a reprogrammable e-fuse 10 inaccordance with an embodiment of the present invention is depicted inFIG. 9. As shown, the e-fuse 10 and a reference element R_(ref) having aresistance R_(ref) serve as the load for cross-coupled nMOS devices N1and N2. A control pin “Sample” is used to activate the sensingoperation. The “Sample” signal is tied to the gate of a PMOS device P1and a nMOS device N3. After programming, most of the current will flowthrough N1 and cause node B to go “high,” since the resistance of e-fuse10 after programming should be substantially higher than that of thereference element R_(ref). The final “high” state is latched in a latchregister 52. On the other hand, after reprogramming, the resistance ofthe e-fuse 10 should be substantially lower than that of the referenceelement R_(ref), and more current will flow through N2 and cause node Bto go “low.” The final “low” state is latched in the latch register 52.During programming and reprogramming, the “Sample” signal is off, sothat both node A and B will be at float. Then, as shown in FIG. 8,either N12 (program) or N13 (reprogram) is on and a path to ground isprovided to allow current to flow only through the e-fuse 10.

The sensing circuit 50 does not allow the reference element R_(ref) tobe shared among a plurality of e-fuses 10. If the size of the referenceelement R_(ref) is relative small this approach is acceptable—a separatereference element R_(ref) can be provided for each e-fuse 10. Otherwise,a sensing circuit 60 such as that shown in FIG. 10 can be used, where asingle reference element 62 in a reference unit 64 is shared by aplurality of e-fuse units 66 (only one is shown). In sensing circuit 60,the reference unit 64 is used to generate a reference voltage equal to(Vdd−I*R_(ref)), where I is the current flow through the reference pathformed by PMOS device P51 and nMOS devices N51 and N61, and R_(ref) isthe resistance of the reference element R_(ref). The current I ismirrored by a shared current source 68.

Each e-fuse unit 66 includes an e-fuse 70. An identical amount ofcurrent I is mirrored via nMOS device N62. The output voltage at node Bis Vdd−I*R_(f), where R_(f) is the resistance of the e-fuse 70. Acomparator 72 is formed by two PMOS devices P53 and P54, two nMOSdevices N52 and N54, and a tail device N63.

The output from the reference unit 64 (node C) is tied to the gate ofthe nMOS device N53 and the output of the e-fuse path (node B) is tiedto the gate of the nMOS device N54. After programming, R_(f)>R_(ref),and the voltage at node B is lower than at node C, so that output of thecomparator 72 will go high and the high state will be latched by latch74. Otherwise, after reprogramming, a low state will be latched by latch74.

An illustrative physical layout of a reprogrammable e-fuse system 80 inaccordance with the present invention is depicted in FIG. 11. As shown,the physical layout of the e-fuse system 80 includes a programming powerand timing generator 82, a reprogramming power and timing generator 84,and a plurality of e-fuse units 86. Each e-fuse unit 86 further includesan e-fuse module 88, a sensing element 90 and a latch 92. Theprogramming power and timing generator 82 and reprogramming power andtiming generator 84 can be merged into a single unit.

As mentioned above, the use of a tapered neck region 16 facilitates theprogramming/reprogramming of the e-fuse 10. An analytical modelillustrating the predicted electro-migration behavior in a taperedstructure is presented below.

The tapered structure 100 used in this analysis is illustrated in FIG.12. As shown, the tapered structure 100 includes first and second bodyregions 102, 104, and a neck region 106 that extends between the firstand second body regions 102, 104. The first and second body regions 102,104 each have a thickness of 0.5 μm, while the thickness of the neckregion 106 varies from a minimum of 0.14 μm to a maximum of 0.5 μm. Thetaper of the neck region 106 is specified by a taper angle β. Based onthe tapered structure 100, the following items were examined:

-   (A) Growth of void during forward current stressing conditions and    void shrinkage during reverse current stressing conditions;-   (B) Effects of taper geometry (β)—vary length of tapered neck region    106, keeping same minimum and maximum widths; and-   (C) Time required to form a void and remove the void for selected    void sizes (e.g., 0.125, 0.25, and 0.5 μm). The following analytic    modeling assumptions were used:-   (A) Void growth emanates from the beginning (i.e., narrowest width    region) of tapered neck region 106 (no incubation time);-   (B) Metal (e.g., Cu) removed from void is deposited at the end of    tapered neck region 106;-   (C) Metal is redeposited in the void during reverse current;-   (D) Void growth kinetics from data on uncapped structures, T=225°    C.;-   (E) Growth velocity has exponential dependence on temperature; and-   (F) Growth velocity has linear dependence on current density.

Based on these modeling assumptions, for a tapered neck region 106 witha length of 1.0 μm, the predicted void growth during forward currentstressing (J₀=70 mA/μm² through 0.5 μm wide line) is illustrated in FIG.13. The predicted void growth and shrinkage for a tapered neck region1.0 μm long and with a taper angle β=10° is illustrated in FIG. 14. Thepredicted void growth and shrinkage for a tapered neck region 106 0.5 μmlong and with a taper angle β=20° is illustrated in FIG. 15. Thepredicted void growth and shrinkage for a tapered neck region 2.0 μmlong and with a taper angle β=5° is illustrated in FIG. 16.

From the above graphs, it can be seen that:

-   (A) Predicted electro-migration behavior in the tapered structure    100 follows asymmetric void growth and shrinkage during forward and    reverse current.-   (B) Taper Angle (β):    -   (1) Larger taper angle β increases time required to reach        equivalent void size and increases nonlinearity of void growth        rate.    -   (2) Total time for void growth and shrinkage is roughly        equivalent.-   (C) Time to reach equal void size is roughly proportional to current    density (J).-   (D) Strong dependence of temperature on void growth (activated    process). (Joule heating not accounted for in analytical model).

FIG. 17 depicts a block diagram of a general-purpose computer system 900that can be used to implement a reprogrammable e-fuse system, an ICincluding a reprogrammable e-fuse system, and the circuit designstructure described herein. The design structure may be coded as a setof instructions on removable or hard media for use by thegeneral-purpose computer 900. The computer system 900 has at least onemicroprocessor or central processing unit (CPU) 905. The CPU 905 isinterconnected via a system bus 920 to machine readable media 975, whichincludes, for example, a random access memory (RAM) 910, a read-onlymemory (ROM) 915, a removable and/or program storage device 955, and amass data and/or program storage device 950. An input/output (I/O)adapter 930 connects mass storage device 950 and removable storagedevice 955 to system bus 920. A user interface 935 connects a keyboard965 and a mouse 960 to the system bus 920, a port adapter 925 connects adata port 945 to the system bus 920, and a display adapter 940 connect adisplay device 970. The ROM 915 contains the basic operating system forcomputer system 900. Examples of removable data and/or program storagedevice 955 include magnetic media such as floppy drives, tape drives,portable flash drives, zip drives, and optical media such as CD ROM orDVD drives. Examples of mass data and/or program storage device 950include hard disk drives and non-volatile memory such as flash memory.In addition to the keyboard 965 and mouse 960, other user input devicessuch as trackballs, writing tablets, pressure pads, microphones, lightpens and position-sensing screen displays may be connected to userinterface 935. Examples of the display device 970 include cathode-raytubes (CRT) and liquid crystal displays (LCD).

A machine readable computer program may be created by one of skill inthe art and stored in computer system 900 or a data and/or any one ormore of machine readable medium 975 to simplify the practicing of thisinvention. In operation, information for the computer program created torun the present invention is loaded on the appropriate removable dataand/or program storage device 955, fed through data port 945, or enteredusing keyboard 965. A user controls the program by manipulatingfunctions performed by the computer program and providing other datainputs via any of the above mentioned data input means. The displaydevice 970 provides a way for the user to accurately control thecomputer program and perform the desired tasks described herein.

FIG. 18 depicts a block diagram of an example design flow 1000, whichmay vary depending on the type of circuit, IC, etc., being designed. Forexample, a design flow 1000 for building an application specific IC(ASIC) will differ from a design flow 1000 for designing a standardcomponent. A design structure 1020 is an input to a design process 1010and may come from an IP provider, a core developer, or other designcompany. The design structure 1020 comprises a circuit 100 (e.g.,reprogrammable e-fuse system, IC including a reprogrammable e-fusesystem, etc.) in the form of schematics or HDL, a hardware-descriptionlanguage, (e.g., Verilog, VHDL, C, etc.). The design structure 1020 maybe on one or more of machine readable medium 975 as shown in FIG. 17.For example, the design structure 1020 may be a text file or a graphicalrepresentation of circuit 100. The design process 1010 synthesizes (ortranslates) the circuit 100 into a netlist 1080, where the netlist 1080is, for example, a list of fat wires, transistors, logic gates, controlcircuits, I/O, models, etc., and describes the connections to otherelements and circuits in an integrated circuit design and recorded on atleast one machine readable medium 975.

The design process 1010 includes using a variety of inputs; for example,inputs from library elements 1030 which may house a set of commonly usedelements, circuits, and devices, including models, layouts, and symbolicrepresentations, for a given manufacturing technology (e.g., differenttechnology nodes, 32 nm, 45 nm, 90 nm, etc.), design specifications1040, characterization data 1050, verification data 1060, design rules1070, and test data files 1085, which may include test patterns andother testing information. The design process 1010 further includes, forexample, standard circuit design processes such as timing analysis,verification tools, design rule checkers, place and route tools, etc.One of ordinary skill in the art of integrated circuit design canappreciate the extent of possible electronic design automation tools andapplications used in design process 1010 without deviating from thescope and spirit of the invention.

Ultimately, the design process 1010 translates the circuit 100, alongwith the rest of the integrated circuit design (if applicable), into afinal design structure 1090 (e.g., information stored in a GDS storagemedium). The final design structure 1090 may comprise information suchas, for example, test data files, design content files, manufacturingdata, layout parameters, wires, levels of metal, vias, shapes, testdata, data for routing through the manufacturing line, and any otherdata required by a semiconductor manufacturer to produce circuit 100.The final design structure 1090 may then proceed to an output stage 1095of design flow 1000; where output stage 1095 is, for example, wherefinal design structure 1090: proceeds to tape-out, is released tomanufacturing, is sent to another design house, or is sent back to thecustomer.

The foregoing description of the preferred embodiments of this inventionhas been presented for purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to aperson skilled in the art are intended to be included within the scopeof this invention as defined by the accompanying claims.

1. A design structure embodied in a machine readable medium used in adesign flow process, the design structure comprising a fuse system, thefuse system comprising: an electrically blowable fuse; circuitry forprogramming the electrically blowable fuse using an electro-migrationeffect; circuitry for reprogramming the electrically blowable fuse usinga reverse electro-migration effect; a reference resistance provided by aportion of the electrically blowable fuse; and circuitry for determininga state of the electrically blowable fuse by comparing a resistance ofthe electrically blowable fuse to the reference resistance.
 2. Thedesign structure of claim 1, wherein the electrically blowable fusecomprises a tapered neck region.
 3. The design structure of claim 1,further comprising: circuitry for adjusting reprogramming conditions tocontrol a shrinkage rate of a void in a neck region of the electricallyblowable fuse.
 4. The design structure of claim 1, wherein the designstructure comprises a netlist, which describes the fuse system.
 5. Thedesign structure of claim 1, wherein the design structure resides onstorage medium as a data format used for an exchange of layout data ofintegrated circuits.
 6. The design structure of claim 1, wherein thedesign structure includes at least one of test data files,characterization data, verification data, or design specifications.
 7. Afuse system, comprising: an electrically blowable fuse; and a system forprogramming the electrically blowable fuse using an electro-migrationeffect, and for reprogramming the electrically blowable fuse using areverse electro-migration effect; wherein the system for programming andreprogramming the electrically blowable fuse comprises: a powergenerator; and a switching circuit coupled to the power generator forselectively applying a programming current and a reprogramming currentto the electrically blowable fuse to program and reprogram theelectrically blowable fuse, respectfully.